I've never seen
e-liquids listed or described in this fashion, but one could assume that they are ready to vape and not require steeping.
Pre-mixed liquids are made in large batches, bottled, and left to steep while in a holding station ready to be shipped. Typically the time from creation to the time it reaches the customer is long enough to be considered steeped.
Custom-mixed liquids are created at the time you make your order. You'll often have the opportunity to request what pg/vg ratio you want and other options like added flavor bumps. These may require some steep time as the time from creation to the time it reaches you will not be as long.
Not all flavors require steeping. Simple single flavors may not. Complex flavors (desert, coffee,
tobacco, or multiple flavor combinations) likely will require steeping.
Pg (propylene glycol) is a better flavor "carrier" than vg (vegetable glycerin). An all vg
e-liquid may require additional time to steep as it is not as good of a flavor carrier; however once allowed to steep it's taste can be as good as a pg/vg mix.